The present invention relates to a focusing ultrasonic transducer element for producing a focusing ultrasonic beam used in an ultrasonic microscope.
In recent years, a mechanical scanning type ultrasonic microscope has been developed in which a microscopic or macroscopic structure and acoustic properties of a substance are observed and measured by using a focusing ultrasonic beam. In the principle of the ultrasonic microscope of this type, a conically focused ultrasonic beam is irradiated on specimen and the focus position of the ultrasonic beam is laterally or vertically shifted on the surface of the specimen, and then the reflected or passed waves of the ultrasonic beam produced by differences between elastic properties of respective points in the specimen are detected and converted to electric signals by an ultrasonic transducer. The image of the ultrasonic microscope is obtained by two-dimensionally displaying the electric signals on a cathode ray tube or by recording these signals in an X-Y recorder. A transducer for forming a focusing ultrasonic beam is typically of a lens system or a system in which an ultrasonic transducer is formed on a convex or concave spherical surface and so on. Also, the ultrasonic microscope is classified into a passing type and a reflection type by the arrangement of the ultrasonic transducer.
In such ultrasonic microscope, as typical means for forming a focusing ultrasonic beam, an acoustic lens system (see Japanese Patent Application No. 18446/1974) or a system in which an ultrasonic trnsducer is constructed on a convex or concave spherical surface (see Japanese Patent Application Nos. 999953/1976, 127498/1976 and 155766/1980) and so on have been practicably used. In the acoustic lens system, a plane sound wave produced in the ultrasonic transducer propagates through an acoustic lens medium, is focused by its acoustic lens portion and is irradiated through an acoustic medium to a specimen. A focal length (F) in this lens system is approximately represented by the following fomula: EQU F=R/(1-C)
where R is the radius of curvature of the sphere in the lens system and C=V.OMEGA./Vl, where V.OMEGA. is the velocity of sound in the acoustic medium and Vl is the velocity of sound of the longitudinal wave in the lens material. A resolving power is approximately proportional to .lambda..multidot.F/D, where .lambda. is the wave length of the sound wave in the acoustic medium and D is the diameter of the lens aperture. When a material having high velocity of sound is selected as an acoustic lens medium, the focal length of the ultrasonic beam becomes short, the ultrasonic beam is made narrow in the focal point and the high resolving power of the ultrasonic microscope is expected, but, because the value of its velocity of sound is finite a spherical aberration is produced in the lens system. Usually, Z-cut sapphire {V.OMEGA.=1483m/s(20.degree. C.)} in which the attenuation in the propagation of the ultrasonic beam is very small and its velocity of sound is high is practically used as a lens material. When the sapphire and water {V.OMEGA.=1483m/s(20.degree. C.)} as an acoustic medium are used, C becomes about 0.133, F does not almost correspond with R and a beam waist W of the focusing ultrasonic beam becomes larger than that of the focusing ultrasonic beam which is obtained in the ideal state of F=R and the resolving power is lowered.
There is a fused quartz (SiO.sub.2) as a cheap lens material in which the attenuation in propagation of a high frequency ultrasonic wave is relatively small. However, the velocity (Vl) of sound of the fused quarts (SiO.sub.2) is 5973m/s, the focal length (F) becomes larger than the radius (R) by 33% and the resolving power becomes lower.
As stated above, because the material having a low velocity of sound makes the focal length F long and makes the resolving power low, the propagating length in an accoustic medium becomes long and signals detected by an ultrasonic transducer become small due to the attenuation in propagation of an ultrasonic wave. The use of the material such as fused quartz is limited for the lens material having a high resolving power.
On the other hand, in the focusing ultrasonic transducer element in which an ultrasonic transducer is arranged on a convex or concave spherical surface, the ultrasonic wave produced by the ultrasonic transducer is ideally converged to one point. In other words, because the focusing ultrasonic transducer element is constructed such as a concentric sphere and the ultrasonic wave is perpendicularly incident upon a boundary surface of the acoustic medium, there is feature that the ultrasonic wave is converged to the center of the concentric sphere without aberration.
However, it is technically not always easy to uniformly form on the concentric sphere an ultrasonic transducer which is efficiently driven.
Also, in the focusing ultrasonic transducer element in which the ultrasonic transducer is supported by a substrate, because the substrate for supporting the ultrasonic transducer is thin and is operated in a composite resonating mode, a frequency band which can be efficiently used is narrow and a response to pulses is low. Furthermore, in a transducer for superhigh frequency, because the size of this transducer and the radius of a spherical surface for mounting it become very small and a thinner substrate becomes necessary for the transducer, the strength of the substrate becomes weak and the transducer cannot be adapted for the high frequency.
In a focusing ultrasonic transducer element having a very thick substrate, the ultrasonic waves can not be efficiently generated and detected, because the area of an ultrasonic transducer becomes too large and the electrostatic capacity of a transducer become large for generating a focusing ultrasonic beam having a wide aperture angle.